Micromechanical systems are devices formed from small components configured on a substrate for example, a silicon substrate.
A MEMS resonator comprises a beam (or spring) structure supported by one or more supports which supports are connected to, or formed integrally with the substrate. MEMS resonators may be used, for example, as parts of clocks or as signal filtering circuits.
A problem with MEMS resonators is that the oscillation frequency of the resonator varies with the ambient temperature and with the materials and processes used to form the resonator.
The main cause of the temperature dependence of the resonance frequency is the temperature coefficient of the elastic (Young's) modulus. Usually a MEMS resonator will be formed from a material having a negative temperature coefficient of the elastic modulus of that material. This means that the spring constant of the resonator will be reduced at high temperatures which in turn results in a reduced oscillation frequency.
Known techniques for attempting to correct the temperature dependence of the oscillation frequency maybe classified as either active or passive temperature compensation techniques.
Known active temperature compensation techniques include temperature measurement followed by bias voltage correction, phase-locked loop (PPL) control feedback, and the heating of resonator springs to a predetermined temperature. A problem with such active temperature compensation techniques is that they suffer from large power dissipation. Additionally in the case of phase-locked loop control feedback, there is an unacceptable phase noise produced.
A known passive compensation technique involves depositing a coating of one or more layers onto the beam of a MEMS resonator. Each of the one or more layers has a different temperature coefficient of the respective elastic modulus to that of the beam. When, as is usually the case, the beam is formed from a material having a negative temperature coefficient of the elastic modulus, then the one or more materials which are deposited or coated onto the beam are chosen because they each have a positive temperature coefficient of the respective elastic modulus. This means that any expansion or contraction of the beam may be compensated for by contraction or expansion of the one or more other materials.
International patent application No. WO 2004/095696 describes a flexural resonator beam in which different materials are used to compensate for the coefficient of thermal expansion. The resonator disclosed in WO 2004/095696 relies on tensional forces in order to compensate for the temperature dependence of the resonant frequency.
A problem with these known passive techniques is that the formation processes such as deposition and oxidation that are used to coat the beam with one or more materials are not well enough controlled to deliver the required accuracy of thickness of coating. In addition, the spring constant is extremely sensitive to the width of the beam which also makes it typically very difficult to obtain accuracy.